This disclosure generally relates to heat exchangers and, more specifically, relates to cross-flow heat exchangers used in conjunction with aircraft environmental control systems.
Traditionally, pressurized aircraft have an environmental control system (ECS) for maintaining cabin pressurization and controlling cabin temperatures during flight. In order to maintain pressurization and control temperature, outside air is supplied to the cabin via air conditioning packs and a portion of the air in the cabin is recirculated by recirculation fans to provide an acceptable level of volumetric airflow to the aircraft passengers.
It is known to supply pressurized air to an ECS using a compressor section of a gas turbine engine. This pressurized air is commonly called “bleed air” and is bled from bleed ports located at various stages of compression in a multi-stage compressor section of the engine. To supply sufficient bleed air over the operating range of the aircraft, typically a high-pressure bleed port is used. The temperature of this bleed air is normally too high for the ECS and some precooling of the bleed air is required.
It is known to install (e.g., inside the engine nacelle) a cross-flow, air-to-air heat exchanger, called a precooler, for cooling hot air bled from a compressor of a gas turbine engine. That cooled air is then supplied to the aircraft ECS. In accordance with a known system, the hot bleed air is cooled in the precooler by cold air diverted from and then returned to the fan duct.
Precooled air for the ECS travels through air conditioning packs to provide essentially dry, sterile, and dust-free conditioned air to the airplane cabin. This conditioned air is then mixed with a predetermined amount of cabin recirculated air and delivered to the aircraft cabin. Trim air, taken downstream of the precooler, may be added to warm the conditioned air to a suitably comfortable level for the aircraft cabin.
For a given volume set of constraints (e.g., volume, maximum pressure drops, etc.), it is desirable to increase the heat transfer capacity of a precooler. That increased heat transfer capacity would allow the designer to reduce the heat exchanger volume (and weight) or achieve higher performance. In either case, the fuel consumption penalty attributable to the extracted bleed air can be reduced.
For example, FIG. 2 illustrates a typical precooler construction. This typical precooler 10 is a cross-flow air-to-air heat exchanger comprising a stack of N rows of air passages, where N is an odd integer equal to 3 or more. If the stacked rows of passages were to be numbered from 1 to N, starting at the bottom of the precooler, then it can be seen in FIG. 2 that the odd-numbered rows are aligned in a first direction indicated by the arrow labeled “COLD FLUID”, while the even-numbered rows are aligned in a second direction indicated by the arrow labeled “HOT FLUID”. (Alternatively, there are some precooler constructions with odd-numbered rows on the hot fluid side and even-numbered rows on the cold fluid side.) In the construction shown in FIG. 2, the second direction is perpendicular to the first direction.
Each air passage seen in FIG. 2 has openings at both ends and a constant cross-sectional area along its length. In a well-known manner, the openings of the odd-numbered rows of passages 12 located on one side of precooler 10 which is visible in FIG. 2 (hereinafter “cold air front side”) are in fluid communication with a source of cold air, while the openings of the even-numbered rows of passages 14 located on the other side of precooler 10 which is visible in FIG. 2 (hereinafter “hot air front side”) are in fluid communication with a source of hot air. For the purpose of discussion, passages 12 will be referred to as “cold passages” and passages 14 will be referred to as “hot passages” to reflect the difference in temperatures of the air flowing through those passages.
In accordance with the construction shown in FIG. 2, the passages within any row have the same height. A person skilled in the art will recognize that the height of cold passages 12 may be different than the height of hot passages 14. The rows of cold and hot passages are arranged so that each row of hot passages 14 is sandwiched between a row of cold passages 12 disposed directly above and a row of cold passages 12 disposed directly below. (Alternatively, each row of cold passages could be sandwiched between respective hot passages above and below.) Adjacent rows of hot and cold passages are thermal-conductively coupled to each other by means of respective rectangular planar parting plates disposed inside precooler 10 in mutually parallel relationship. A precooler with N rows of stacked hot and cold passages has (N−1) parting plates.
The parting plates are rigidly supported in a mutually parallel relationship by a frame that comprises a multiplicity of pairs of mutually parallel cold passages closure bar 18 (a respective pair of cold passages closure bar flanking each row of cold passages), a multiplicity of pairs of mutually parallel hot passages closure bar 20 (a respective pair of hot passages closure bar flanking each row of hot passages) oriented perpendicular to the cold passages closure bar and having ends interleaved between the ends of cold passages closure bar 18, and a pair of side plates 22 and 24 which are respectively affixed to the outermost (first and last) pairs of cold passages closure bar 18. (In other embodiments, the side plates could be affixed to outermost pairs of hot passages closure bars.) The side plates 22, 24 are disposed parallel to the parting plates and adjacent to the first and N-th rows of air passages, which in the depicted construction are cold passages.
During operation of precooler 10 shown in FIG. 2, cold air flows through cold passages 12 and hot air flows through hot passages 14, which results in the transfer of heat from the hot air to the cold air by thermal conduction. The heat exchanger thus extracts heat from the hot air to lower its temperature to the degree required by the particular application.
Still referring to FIG. 2, it is known to form each row of air passages using a multiplicity of mutually parallel fins which extend between a respective pair of adjacent parting plates. In FIG. 2, fins 26 partly define cold passages 12, while fins 28 partly define hot passages 14. In accordance with the construction depicted in FIG. 2, fins 26 are spaced at equal intervals within each row of cold passages 12 (i.e., the rows of cold passages have a constant fin density), while fins 28 are spaced at equal intervals within each row of hot passages 14 (i.e., the rows of hot passages have a constant fin density).
In yet another example of the prior art, FIG. 3 shows the hot air front side of a precooler having constant fin density, i.e., the view in FIG. 3 is taken on the side where hot air enters the hot passages. Each row of hot passages 14 comprises a corrugated sheet 30 made of metal or metal alloy which is placed between a pair of parting plates 16. The corrugated metal sheet 30 is formed by folding. Each corrugated metal sheet 30 is made of a corrosion-resistant metal or metallic alloy having a high thermal conductivity.
As seen in FIG. 3, each corrugated metal sheet 30 has three types of corrugated sheet segments: passage top segments 32, passage bottom segment 34, and fins 28 which connect passage ceiling segments 32 to passage floor segments 34. In the case of a particular pair of adjacent hot passages 14a and 14b shown in FIG. 3, the first hot passage 14a is formed by fins 28a, 28b, a passage top segment 30a connecting fins 28a, 28b, and a portion of a lower parting plate 16a that opposes passage top segment 30a across hot passage 14a, whereas the second hot passage 14b is formed by fins 28b, 28c, a passage bottom segment 34a connecting fins 28b, 28c, and a portion of the upper parting plate 16b that opposes passage bottom segment 34a across hot passage 14b. 
Preferably, all of the passage top segments 32 in each row of hot passages are brazed to the bottom surface of a respective parting plate disposed above the row, while all of the passage bottom segments 34 in each row of hot passages are brazed to the top surface of a respective parting plate disposed below the row. The preferred brazing material has high thermal conductivity, thereby facilitating the transfer of heat at the interface between a parting plate and a passage top or bottom segment. The above-described corrugated structure is repeated across each row for all rows of hot passages. The rows of cold passages (not visible in FIG. 3) may have a similar construction.
Referring again to FIG. 3, a heat exchanger is depicted in which the fin density across each row of hot passages is constant. Likewise the fin density across each row of cold passages is constant.
There is a need for improvements to ECS precoolers that increase the precooler's heat transfer capacity for a given set of volume constraints.